Lisianthus, Eustoma grandiflorum of the Gentianaceae family, is a relatively new floral crop to the international market, which is widely used as cut flower and also as potted plant. Common names for the naturally occurring plant are Texas Blue Bell, Prairie Rose and Prairie Gentian. Lisianthus is a diploid organism with ability for self and cross-pollination and almost all the seed in the commercial market are F1 hybrids. The species E. grandiflorum originates from the prairies of the plain states in Northern America and is described as an annual or biennial that flowers in spring or early summer. The only other known species in the genus Eustoma is E. exaltatum, which is capable of cross pollination with E. grandiflorum. In a period of less than 30 years starting in the late 20th century lisianthus shifted from being practically anonymous plant to being one of the top 10 cut flower crops in the world.
Attempts to transform the wild type bedding phenotype into the modern cut flower crop are going back to as early as 1930s and were mainly taken in Japan. However, a significant breakthrough occurred only in 1977 with the development of the first F1 hybrids that were introduced to the international market around 1984 as a series of varieties by the name “Yodel”. Breeding programs aiming at improved varieties for pot or bedding plants or for the cut flower market have started at late 1980s. Today, the lisianthus market is mainly focused on cut flower varieties.
The introduction of lisianthus as a crop plant has been slow, encountering cultivation and economical challenges including inadequate growth pattern, low flower yield, lack of uniformity and long growth period. The rising interest in this crop has lead to a parallel increase in scientific literature; however, it is still considered a floriculture example of an “orphan crop”, a crop that is economically and culturally important but lacking a substantial research interest.
A limited work has been dedicated to examine trait inheritance in lisianthus, none of which combined molecular information and inheritance mechanism. Ecker et al. (Ecker R et al., 1993. Genet. Anal. 256: 253-257; Ecker R et al 1994. Euphytica 78: 193-197) showed a clear heterotic effect on growth rate, leaf size, stem diameter and number of nodes. The experiments were conducted on different inbred, F1, F2 and BC1 populations with a wide genetic background. A model for seed dormancy inheritance was proposed based on an analysis of F1, F2 and BC1 populations originated from a cross between genotypes requiring and genotypes not-requiring cold temperature for flowering. The model includes six diallelic loci with cumulative effects, in which the presence of at least nine ‘dormancy-conferring’ alleles is necessary for inducing phenotypic seed dormancy (Ecker R et al 1994. Plant Breed. 113: 335-339).
Lisianthus is considered a facultative long-day plant and although the photoperiod effect is considered to be minor, experiments have shown that a short day can have a delaying effect on flowering and also have a negative secondary effect on bolting (Harbaugh B K., 1995. HortScience. 30: 1375-1377).
The main environmental factor influencing growth and flower induction in lisianthus is temperature (Ohkawa K and Sasaki E., 1999. Acta Hortic. 482: 423-426). When lisianthus seedlings are exposed to temperatures above 20° C. for more than 14 days in the first growing stages, rosette growth occurs and elongation of flowering stem is delayed. Plants with rosette leaves will not bolt and flower very late in a scattered manner which is incompatible with agricultural production. Exposure to low temperature below 15° C. for at least 4 weeks (a process called “vernalization”) has been proven to eliminate the negative effect of the high temperature (Ohkawa K et al., 1991. Sci. Hortic. (Amsterdam). 48: 171-176). Gibberellic acids (GAs) were found to play an important role in the vernalization effect by regulating stem elongation in a number of plants including lisianthus (Hisamatsu T et al., 1998. J. Japanese Soc. Hortic. Sci. 67: 866-871). Low temperatures can initiate GA biosynthesis and elevate GA sensitivity in vegetative rosette in Arabidopsis thaliana and lisianthus (Oka M et al., 2001. Plant Sci. 160: 1237-1245). Reduced glutathione (GSH) also has been shown to have a role in the response to vernalization in lisianthus, presumably by affecting regulation of bolting upstream of GA (Yanagida M et al., 2004. Plant Cell Physiol. 45: 129-37).
Lisianthus homologous of a few well-known genes that may play a role in the vernalization requirements have been investigated (Nakano Y et al., 2011. Physiol. Plant. 141: 383-93). The genes were selected based on their function in Arabidopsis vernalization mechanism. FLOWERING LOCUS C (FLC) encodes a MADS-box transcription factor and is a key repressor of flowering that is repressed by vernalization. FLOWERING LOCUS T (FT) and OVEREXPRESSION OF CONSTANS 1 (SOC1) are floral promoters that are suppressed by FLC. Function of the homologous lisianthus genes was examined by overexpressing the Eustoma genes in transgenic Arabidopsis plants. Expression analysis in different tissues and times for vernalized and non-vernalized plants indicated that EgFLCL (E. grandiflorum FLC-like) is up-regulated by cold temperatures and therefore defers from Arabidopsis FLC that is abundantly expressed before cold treatment and is silenced by vernalization. EgSOC1L (E. grandiflorum SOC1-like) and EgFTL (E. grandiflorum FT-like) were induced by warm temperatures and long day conditions following vernalization in a similar pattern to that observed for Barley Hv-FT1 which is induced by warm and long day conditions following vernalization (Hemming et al., 2008). These findings suggest that flowering regulation by vernalization in Eustoma differs considerably from the paradigm developed for Arabidopsis thaliana (Nakano et al., 2011, ibid).
The requirement to expose young seedling of lisianthus to temperatures below 15° C. for at least 4 weeks is a burden imposed on growers in terms of time and money, particularly in worm weather countries like Israel that are otherwise highly suitable for the production of lisianthus cut flowers.
Thus, there is still an unmet need for and it would be highly advantageous to have lisianthus (Eustoma grandiflorum) plants that are insensitive to vernalization and do not require cold treatment for bolting and flowering, while keeping the phenotypes of commercially successful lines.